1. Field of the Invention
The present invention relates to an electronic element, a variable capacitor, a micro switch, a method for driving the micro switch, and an MEMS type electronic element. As for the variable capacitor, an actuator is used. The variable capacitor is usable, for example, for a wireless communication apparatus, an RF measuring apparatus, etc. The micro switch is exemplified by the so-called MEMS (Micro-Electro-Mechanical System) switch, or the like. The micro switch includes, for example, a high frequency switch (RF switch) to be used, for example, for a wireless communication apparatus or an RF measuring apparatus, and a switch for the DC signal or the low frequency signal.
2. Description of the Related Art
The wireless communication or radio communication technique, which uses the mobile phone device, etc., is advanced, and the importance is enhanced for the variable capacitor and the micro switch to be used, for example, for the high frequency circuit.
Conventionally, a varactor, which is a semiconductor device, is used as the variable capacitor to be used for the high frequency circuit, etc. However, the Q value of the varactor is small, and various inconveniences arise.
“A micromachined variable capacitor for monolithic low-noise VCOs” by Darrin J. Young and Bernhard E. Boser, Solid-State and Actuator Workshop, Hilton Head, June 1996, pp. 86-89, which is a non-patent document (general scientific or technical document) discloses a variable capacitor which is formed by using the technique of MEMS (Micro-Electro-Mechanical System) as the variable capacitor having a large Q value. The variable capacitor includes a fixed electrode and a movable electrode which are arranged to construct plane-parallel. A movable portion is supported by a support portion having a flexible structure. The movable portion is movable toward the fixed electrode from a position separated (away) from the fixed electrode by a predetermined spacing distance (initial spacing distance). By the movement of the movable portion, the capacitance or electric capacity of the variable capacitor is changed.
When the driving voltage is applied between the both electrodes, the movable portion is moved toward the fixed electrode by the electrostatic force. When the movable portion is moved, then the support portion having the flexible structure is warped or flexibly bent to thereby generate the spring force. The spring force is generated in a direction so as to return the spacing distance between the both electrodes to the predetermined spacing distance (initial spacing distance). Therefore, the both electrodes are stabilized at the point at which the electrostatic force and the spring force are balanced with each other, and the capacitance, which is to be used as the output, is formed. That is, the both electrodes function as the capacitance electrodes, and function also as the driving electrodes for generating the electrostatic force resisting against the spring force in order to adjust the spacing distance between the both electrodes. In the variable capacitor, the capacitance is changed (controlled) in accordance with the balance between the spring force and the electrostatic force. Therefore, the variable capacitor has such an advantage that the electric power consumption is small.
However, the variable capacitor disclosed in “A micromachined variable capacitor for monolithic low-noise VCOs” has a low variable capacitance ratio (capacitance change ratio). The reason thereof will be explained below.
In the variable capacitor disclosed in “A micromachined variable capacitor for monolithic low-noise VCOs”, when the movable electrode is driven, the movable electrode is stopped at a position at which the electrostatic force between the fixed electrode and the movable electrode is balanced with the spring force. The spring force is proportional to the amount of change of the spacing distance between the both electrodes from the initial spacing distance. As the spacing distance between the both electrodes is narrower, the spring force becomes greater. On the other hand, the electrostatic force is proportional to the square of the voltage between the both electrodes, and the electrostatic force is inversely proportional to the square of the spacing distance between the both electrodes. As the spacing distance between the both electrodes is narrower, the electrostatic force becomes greater. That is, when the spacing distance between the both electrodes is changed, the spring force is changed proportionally to the amount of change, while the electrostatic force is changed inversely proportional to the square of the amount of change. Therefore, the spring force and the electrostatic force can be balanced with each other within a certain limited spacing distance range, but the spring force and the electrostatic force cannot be balanced with each other at any position exceeding the range.
In the case of the variable capacitor disclosed in “A micromachined variable capacitor for monolithic low-noise VCOs”, the spring force and the electrostatic force can be stably balanced with each other by the voltage applied between the both electrodes until the spacing distance therebetween arrives at a spacing distance which is about ⅓ of the initial spacing distance. The spring force and the electrostatic force can be stably balanced with each other within a range from the initial spacing distance of the spacing distance between the both electrodes to the spacing distance of about ⅓ of the initial spacing distance. The movable electrode can be stably stopped at any spacing distance between the electrodes, within the above spacing distance range, depending on the applied voltage.
In the variable capacitor disclosed in “A micromachined variable capacitor for monolithic low-noise VCOs”, however, when the voltage applied between the both electrodes is increased and when the spacing distance between the both electrodes becomes narrower than the spacing distance of about ⅓ of the initial spacing distance, then the spring force and the electrostatic force cannot be stably balanced with each other. That is, in relation to positions at each of which the spacing distance between the both electrodes is narrower than the spacing distance of about ⅓ of the initial spacing distance, the electrostatic force exceeds the spring force at each of such positions. As a result, the movable electrode is moved to a limit position at which the movable electrode approaches closest to the fixed electrode, irrelevant to the magnitude of the voltage (voltage magnitude) at any driving voltage of not less than the voltage at which the spacing distance between the both electrodes is the spacing distance of about ⅓ of the initial spacing distance. The so-called pull-in phenomenon, in which the spacing distance cannot be controlled by the voltage, consequently arises in the variable capacitor. The minimum voltage, at which the pull-in phenomenon begins to arise, is hereinafter referred to as “pull-in voltage”.
Therefore, in the variable capacitor disclosed in “A micromachined variable capacitor for monolithic low-noise VCOs”, the spacing distance between the both electrodes can be controlled by the driving voltage only within the range of the spacing distance in which the spacing distance between the both electrodes is not more than about ⅓ of the initial spacing distance. That is, in the variable capacitor disclosed in “A micromachined variable capacitor for monolithic low-noise VCOs”, a range (continuous adjustable range), in which the spacing distance between the both electrodes can be continuously adjusted by the voltage applied between the both electrodes, is practically limited to the range from the initial spacing distance to the spacing distance of about ⅓ thereof.
In the variable capacitor disclosed in “A micromachined variable capacitor for monolithic low-noise VCOs”, the fixed electrode and the movable electrode are not only used as the driving electrodes for adjusting the spacing distance therebetween, but are used also as the capacitance electrodes for forming the capacitance to be outputted. The capacitance, which is formed between the both electrodes, is inversely proportional to the spacing distance between the both electrodes. When the spacing distance between the both electrodes is decreased, the capacitance is increased. As described above, the spacing distance between the both electrodes can be controlled within the range from the initial spacing distance (widest spacing distance) to the spacing distance of about ⅓ thereof. Therefore, the variable capacitor disclosed in “A micromachined variable capacitor for monolithic low-noise VCOs” can be controlled, by the control based on the driving voltage, until arrival at a capacitance which is about 1.17 times the capacitance in the initial state, provided that the capacitance in the initial state is 1. The variable capacitance ratio (capacitance change ratio) is extremely low, for example, as compared with the variable capacitance ratio (capacitance change ratio) to be obtained if the control is made with the same or equivalent electrode control range from a spacing distance between the both electrodes which is substantially zero.
On the other hand, a switch, which is a semiconductor device including PIN-DIODE, MOS-FET, etc. has been hitherto used as the micro switch.
However, as the wireless communication technique is advanced in recent years, the frequency used in the wireless communication technique is in a high band of several GHz to several 10 GHz. In the high frequency band, the transmission loss, which results from the low Q of the switch which is the conventional semiconductor device, becomes a problem. In future, there is a possibility that, for example, the mobile or portable terminal is required to cover various frequency bands with one mobile or portable terminal. In such a situation, it is postulated that the number of the switches to be used to switch or select the frequency band would be increased. In such usage, a problem arises such that the switch which is a semiconductor device, consumes high electric power.
In view of the above, MEMS switches, which are constructed variously, have been suggested as the RF switch. The MEMS type RF switch performs the switching in accordance with the MEMS technique. Therefore, it is possible to suppress the transmission loss to be sufficiently low. The MEMS type RF switch can be classified into the so-called serial coupling resistance type switch (hereinafter referred to as “serial type switch”) and the parallel coupling capacitance type switch (hereinafter referred to as “parallel type switch”) based on the way of connection or wiring with respect to the transmission line path.
The serial type switch is disclosed, for example, in Japanese Patent Application Laid-open No. 5-2976. A serial type switch is connected in series to one transmission line path. The serial type switch functions as the switch such that the electric contact of the fixed portion and the electric contact of the movable portion make contact with each other or separation from each other. The serial type switch can be used not only as the RF switch, but the serial type switch can be also sued as the switch for the DC signal or the low frequency signal.
The parallel type switch is disclosed, for example, in Japanese Patent Application Laid-open No. 2004-6310. A parallel type switch has the capacitance, and the capacitance is connected in parallel to a pair of high frequency transmission line paths. By changing the distance of the movable portion from the fixed portion in the parallel type switch, the capacitance of the parallel type switch is changed, and the transmission signal band is changed. The inputted high frequency signal is shunted or not shunted to the ground conductor depending on the magnitude of the capacitance. The state, in which the high frequency signal is shunted to the ground conductor, corresponds to the OFF state of the switch. The state, in which the high frequency signal is not shunted to the ground conductor, corresponds to the ON state of the switch.
The MEMS switch adopts the electrostatic driving system in which the movable portion is driven by the electrostatic force in order to perform the switching operation. Therefore, the MEMS switch is an electronic element in which the electric power consumption is low.
However, the conventional MEMS type RF switch, which adopts the electrostatic driving system, requires the high driving voltage, even when the conventional MEMS type RF switch is either the serial type switch or the parallel type switch. The reason for this will be explained below while being divided into the case of the serial type switch and the case of the parallel type switch. Note that the serial type switch requires high driving voltage, either when the serial type switch is used as the RF switch or when the serial type switch is used as the switch for the DC signal or the low frequency signal.
The conventional serial type switch described in Japanese Patent Application Laid-open No. 5-2976, which adopts the electrostatic driving system, has a substrate and a movable portion. The movable portion is held by a support portion in a state that the movable portion is separated from the substrate. The substrate has a fixed driving electrode and a fixed electric contact at a portion of the substrate, the portion being opposite to or facing the movable portion. The movable portion has a movable driving electrode and a movable electric contact at a portion of the movable portion, the portion being opposite to or facing the substrate. When the voltage is applied, the electrostatic force is generated between the both driving electrodes so that the spacing distance between the both contacts is narrowed. The movable portion is displaced by the electrostatic force, and the movable portion functions as a plate spring. When a predetermined voltage is applied between the both driving electrodes so that the electrostatic force, which is greater than the restoring force of the plate spring, is generated, then the both contacts make contact with each other against the restoring force of the plate spring, and the switch is in the ON state.
The spring force of the movable portion is generated in accordance with the Hooke's law, which is proportional to the amount of change from the initial spacing distance in relation to the spacing distance between the both driving electrodes. On the other hand, the electrostatic force is proportional to the square of the voltage between the both driving electrodes, and the electrostatic force is inversely proportional to the square of the spacing distance between the both driving electrodes. It is necessary that the pull-in voltage or a voltage of not less than the pull-in voltage is applied, between the both driving electrodes of the serial type switch, as the voltage which makes it possible to allow the both driving electrodes to make contact with each other against the restoring force of the plate spring.
On the other hand, when the voltage is not applied between the both driving electrodes, then the electrostatic force is not generated between the both driving electrodes, and thus the movable portion is returned to the initial position in accordance with the restoring force of the spring, thereby separating the both contacts from each other and turning the switch to the OFF state. As described above, in order to perform the switching between the ON state and the OFF state of the switch by controlling the voltage applied between the both driving electrodes, it is necessary that the voltage between the both driving electrodes should be switched between no voltage and the pull-in voltage or the voltage of not less than the pull-in voltage.
In the conventional serial type switch as described above, the passing loss (loss caused by the contact resistance) of the signal between the both electric contacts is generated in the state (ON state) in which the electric contacts make contact with each other. In order to reduce the passing loss, it is necessary that the contact pressure between the both contacts is increased. In general, a technique is adopted, in which the contact pressure between the both contacts is increased by increasing the electrostatic force in the ON state.
When the ON state is released to the OFF state, the movable driving electrode is not separated from the fixed driving electrode at the pull-in voltage. The movable driving electrode is separated from the fixed driving electrode at a voltage smaller than the pull-in voltage by the spring force (opening or releasing spring force) obtained in that situation. The applied voltage-displacement curve of the movable driving electrode is a hysteresis curve in which the open circuit voltage is lower than the applied voltage.
Therefore, if the spring constant of the plate spring is decreased in order to decrease the pull-in voltage, any opening spring force, which is sufficient to separate the movable driving electrode from the fixed driving electrode, is not obtained consequently. If any sufficient opening spring force is not obtained, the movable electric contact is not separated from the fixed electric contact. Therefore, the serial type switch does not function as the switch.
Therefore, in order to reliably open the electric contacts, it is necessary that the restoring force of the spring of the movable portion is increased to some extent. As described above, in the conventional serial type switch, in view of the signal pass characteristic (low loss) and the operation reliability, it is necessary to make the design so that the pull-in voltage has a magnitude of some extent. Further, it is necessary that the driving voltage for allowing the switch to be in the ON state is not less than the pull-in voltage.
The conventional parallel type switch, which adopts the electrostatic driving system, has a substrate and a movable portion. The movable portion is stacked on the substrate with a dielectric intervening therebetween. The movable portion is supported while being separated from the substrate so that the movable portion functions as the plate spring. A fixed driving electrode and a fixed capacitance electrode are provided at a portion, of the substrate, which is opposite to or faces the movable portion. The movable portion has a movable driving electrode and a movable capacitance electrode at a portion, of the movable portion, which is opposite to the substrate. When the voltage is applied between the both driving electrodes, then the electrostatic force is generated, and the spacing distance between the both capacitance electrodes is narrowed. The capacitance, which is brought about by the both capacitance electrodes, is provided in parallel to the high frequency transmission line path. When a predetermined voltage (pull-in voltage), which generates the electrostatic force greater than the restoring force of the plate spring, is applied between the both driving electrodes, then the spacing distance between the both capacitance electrodes becomes narrowest, thereby maximizing and the capacitance between the both capacitance electrodes. This causes the inputted high frequency signal to be shunted to the ground conductor, and the switch is in the OFF state. On the other hand, when the voltage is not applied between the both driving electrodes, then the electrostatic force is not generated between the both driving electrodes, and the movable portion is returned to the initial position by the restoring force of the spring, thereby increasing the spacing distance between the both capacitance electrodes and decreasing the capacitance brought about by the both capacitance electrodes. The inputted high frequency signal is transmitted without being shunted to the ground conductor, and the switch is in the ON state. By controlling the voltage between the both driving electrodes as described above, it is possible to switch the ON state and the OFF state of the switch.
In the conventional parallel type switch as described above, as the spacing distance between the both capacitance electrodes in the ON state is greater, the capacitance becomes smaller. Therefore, it is possible to lower the insertion loss by the switch. The capacitance is increased when the spacing distance between the both capacitance electrodes is decreased as narrow as possible in the OFF state. Therefore, it is possible to enhance the cutoff characteristic (isolation) of the high frequency signal. Therefore, in the parallel type switch, it is desirable that the initial spacing distance between the both capacitance electrodes is increased; and that the movable driving electrode is movable to a great extent. These features cause a factor to raise the driving voltage. As described above, in the conventional parallel type switch, in view of the signal pass characteristic, the initial spacing distance between the both driving electrodes has to be designed to be great, and the driving voltage has to be made great.
As described above, in any one of the serial type switch and the parallel type switch, the conventional MEMS type RF switch adopting the electrostatic driving system requires the high driving voltage in view of the signal pass characteristic and the reliability. Therefore, it has been difficult to provide the conventional MEMS type RF switch on the mobile or portable terminal, etc. which is required to be driven at a low voltage of about several V or less. The serial type switch also requires the high driving voltage not only when the serial type switch is used as the RF switch but also when the serial type switch is used as the switch to perform the switching for the DC signal and the low frequency signal.
The present invention has been made taking the foregoing situations into consideration, an object of which is to provide a variable capacitor which is capable of enhancing the variable capacitance ratio while making the capacitance to be variable by utilizing the electrostatic force.
Another object of the present invention is to provide a micro switch which is capable of decreasing the driving voltage without causing any special inconvenience or problem even when the electrostatic drying system is adopted, and a method for driving such micro switch.
Still another object of the present invention is to provide an electronic element which is capable of being driven in a movement range that is equivalent to or not less than the conventional movement range within a range of the driving voltage smaller than the pull-in voltage, as well as a variable capacitor, a micro switch, a method for driving the micro switch, and an MEMS type electronic element.